Field of the Invention
The invention relates generally to coatings to enhance the durability and operating lifetime of downhole tools and other devices. More particularly, the invention relates to high-velocity-oxy-fuel (HVOF) coatings applied to downhole tools and other devices to enhance strength, resistance to abrasive wear, resistance to corrosion, and resistance to spallation and cracking.
Background of the Technology
In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a “drill string”, then rotate the drill bit while applying weight-on-bit to allow the bit to progress downward into the earth along a predetermined path to form a borehole. A typical drill string is made up from an assembly of drill pipe sections connected end-to-end, plus a “bottom hole assembly” (BHA) disposed between the bottom of the drill pipe sections and the drill bit. The BHA is typically made up of sub-components such as drill collars, stabilizers, reamers and/or other drilling tools and accessories, selected to suit the particular requirements of the well being drilled.
The drill string and bit are often rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel). During the drilling process, a drilling fluid (commonly referred to as “drilling mud” or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the annular space (“wellbore annulus”) between the drill string and the wellbore. The drilling fluid carries borehole cuttings to the surface, cools the drill bit, and forms a protective cake on the borehole wall (to stabilize and seal the borehole wall), as well as other beneficial functions. At surface the drilling fluid is treated, by removing borehole cuttings, amongst other possible treatments, then re-circulated by pumping it downhole under pressure through the drill string.
As an alternative to rotation by a rotary table or top drive alone, a drill bit can also be rotated using a “downhole motor” incorporated into the BHA immediately above the drill bit. The technique of drilling by rotating the drill bit with a downhole motor without rotating the drill string is commonly referred to as “slide” drilling. It is common in certain types of well-drilling operations to use both slide drilling and drill string rotation, at different stages of the operation.
The borehole resulting from drilling operations is typically lined with casing that is cemented into place, and then the well is completed to initiate production of hydrocarbon fluids from the reservoir.
During drilling and production operations, various devices, tubulars, downhole tools, and associated hardware are subject to harsh downhole conditions. For example, downhole tools and devices are often exposed to axial and radial impact loads, friction loads from sliding engagement with outer components, high pressures, corrosive fluids, abrasive fluids, or combinations thereof. Such conditions can detrimentally wear and/or decrease the operating lifetime of such tools and devices. Accordingly, specialized coatings, referred to as metal spray coatings, are often applied to the outer surfaces of such tools and devices to protect them from the harsh conditions.
High-velocity-oxy-fuel WC-10Co-4Cr (HVOF) coatings are one type of conventional metal spray coating used on drilling tools. HVOF coatings do provide enhanced protection, however, common failure modes including blistering, spalling, and cracking have been observed in the field.
Accordingly, there remains a need in the art for improved metal spray coating materials for downhole tools and devices. Such metal spray coating materials would be particularly well received if they provided enhanced yield strength, corrosion resistance, and thermal shock resistance as compared to conventional metal spray coatings.